84 - Tach not working correctly

[Dan Owens]

Guys,

After several years of the tach upshift light not working it started to work again all by it's self but now it stopped registering RPM's. Every once in a while the needle starts to register again but then stops in 30 seconds or so. Upshift light works like a champ though, not that it helps any. Any ideas of what it might be? I know that's a stupid question but just looking for a direction to focus if anyone has experience with this problem. The tach hasn't been removed nor has the engine or wiring in quite some time. Think it's time to replace it?

[Dan Owens]

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[ChrisBennet]

Hi Dan,
From memory, to disable the upshift (CASIS) light you remove the black wire. The light will still come on when you hit the red limiter though.

In case you have a scope, this is what the tach signal (black with violet wire?) looks like at 2000 and 4000 RPM:






Disregard the voltage, the signals are actually about 12V.

Hope this helps,
-Chris

[Dan Owens]

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[Early_S_Man]

Dan,

If you remove the tach and take the Black/Violet [TD lead] wire off ... you can also check the TD lead for pulse activity by using a multimeter set to AC Volts ... while the engine is running!

If you don't have the pulses at the tach, try removing the DME connector and cleaning with 90% isopropyl alcohol. Reconnect after thorough drying, and retest for pulse activity at the TD lead with multimeter.